The present invention relates to a power source circuit for use with a battery-driven portable device.
A switching power source is frequently used for the power source of a portable device because it may be reduced in size and weight. The switching power source may be categorized into a PWM (Pulse Width Modulation) converter which is operated by a rectangular waveform signal and an oscillation type converter which is operated by a sinusoidal waveform signal, when categorized on the basis of the waveforms of the switching signal. The PWM converter of the voltage increasing type will be described for the prior art of the present invention.
FIG. 6 is a block diagram of an equivalent circuit of a prior power source circuit. In the figure, reference numeral 1 is an input terminal of the power source circuit connected to a power source (not shown), e.g., a battery; 2 is an element (referred to as a switch), which operates like a switch and functions to connect and disconnect the power source connected to the equivalent circuit to and from the circuit proper of the power source circuit; 3 is an element (referred to as an input capacitor) which operates like a capacitor and functions to stabilize the power source voltage or an input voltage to the circuit proper of the power source circuit; 4 is an element (referred to as an inductance) which operates like an inductance for storing electric energy; 5 is an element (referred to as a transistor) which operates like a switching transistor; 6 is an element (referred to as a diode) which operates like a rectifying diode; 7 is an element (referred to as an output capacitor) which operates like a capacitor for smoothing the rectified waveform of the input voltage; 8 is a control circuit for controlling the output voltage so that it takes a constant voltage value; and 9 is an output terminal connected to a load circuit.
The operation of the prior power source circuit thus constructed will be described. A battery is used for the power source in the power source circuit. A d.c. voltage is applied from the battery to the input terminal 1. The d.c. voltage goes from the input terminal 1 to one of the terminals of the switch 2. The other terminal of the switch 2 is connected to the circuit proper of the power source circuit. When those terminals of the switch 2 are disconnected from each other (viz., the switch 2 is in an open state), no electric power is supplied from the power source to the circuit proper of the power source circuit. When the terminals of the switch 2 are connected to each other (viz., the switch 2 is in a close state), the electric power is supplied from the power source to the circuit proper. When the switch 2 is in a close state, the input d.c. current is fed to the input capacitor 3 to charge the same, while at the same time it is fed to the inductor 4 and its subsequent circuitry. When a load current abruptly increases, the input capacitor 3 is discharged to suppress the resultant variation of the input voltage to stabilize the input voltage. When the transistor 5 is in an on state, the inductor 4 is excited to store energy therein. When the transistor 5 is in an off state, a magnetic flux developed by the inductor 4 is reset and the energy stored in the inductor 4 is discharged to the diode 6 and the subsequent circuitry.
When the cathode potential is lower than the anode potential, the diode 6 is rendered conductive (turned on) to charge the output capacitor 7. When the cathode potential is higher than the anode potential (e.g., when the transistor 5 is turned on), the diode 6 is rendered nonconductive (turned off). In a state that the diode 6 is turned off, the output capacitor 7 is discharged, and the discharging current flows into the load circuit that is connected to the output terminal 9. The control circuit 8 monitors potential at the output terminal 9. Specifically, it compares the potential with a reference potential or voltage, and controls the turning on and off of the transistor 5 on the basis of the comparison result so as to keep the output voltage constant. Further, the control circuit 8 also monitors the current flowing through the transistor 5, and controls the turning on and off of the transistor 5 on the basis of the result of the monitoring so as to restrict the current within a predetermined range of current. The control circuit 8 controls the turning on and off of the transistor 5 by varying the frequency where one cycle is defined by the time period between an on time point and an off time point of the transistor, varying the duty cycle of the time period, interrupting the switching operation, or another suitable method.
Immediately after the switch 2 is turned on, the output voltage is 0 V. Therefore, the control circuit 8 turns on the transistor 5 to increase the output voltage. When the switch 2 is turned on, a current to charge the input capacitor 3 and a current to excite the inductor 4 (the current flowing through the inductor 4 and the transistor 5), which are fed from the power source, simultaneously flow to those elements from the power source. A large current, which is required immediately after the switch 2 is turned on, flows through the power source. As a result, the power source voltage, or the input voltage, greatly drops by the internal resistance of the battery. The input voltage serves also as a power source voltage to the control circuit 8. Accordingly, if the input voltage is greatly reduced, the control circuit 8 possibly fails to operate. If the control circuit 8 is not operated in a state that the transistor 5 is in an on state, the current flowing through the transistor 5 continues. The result is that the input voltage remains low and no output voltage is frequently produced.
An operation of the prior power source circuit thus constructed will be described hereunder. The following specific example will be used for the operation description: the peak rush current to the input capacitor 3 is 1.5 A; the maximum collector current of the transistor 5 is 2 A; the minimum operating power source voltage of the control circuit 8 is 1.8 V; the initial battery voltage is 3.2 V; and the internal resistance of battery is 0.5.OMEGA.. When the rush current peaks, the output voltage of the battery drops by a voltage given below EQU (1.5 A+2 A).times.0.5.OMEGA.=1.75 V
That is, the input voltage drops from its initial voltage by 1.75 V, and the resultant input voltage is 1.45 V: 3.2 V-1.75 V=1.45 V. This input voltage value, 1.45 V, is lower than 1.8 V of the minimum operation power source voltage of the control circuit 8: 1.45 V&lt;1.8 V. Thus, in the prior power source circuit, when the rush current to the input capacitor 3 reaches its peak value immediately after the switch 2 is turned on, the transistor 5 is left turned on, and the control circuit 8 possibly fails to operate.
Since the prior power source circuit is thus constructed, it possibly fails to operate even if the switch 2 is turned on.